FIELD-MEASUREMENTS OF TRANSFORMER SINGLE-PHASE EXCITING CURRENT AS A DIAGNOSTIC-TOOL, AND INFLUENCE OF LOAD TAP CHANGERS

The single-phase exciting-current test can be used to detect undesirab le conditions in single- and three-phase transformers. Normally the te st results are analyzed by comparing currents between all three phases in a given transformer and between the similar single-phase units. Ce rtain problems, however, can be detected on the basis of current chang e when the measurements are performed on different load tap changer po sitions. Accordingly, understanding how the load tap changer affects t he current magnitude of individual phases is essential for developing proper analysis. This paper identifies twelve patterns of test result behavior, and an attempt is made to show different load-tap-changer de signs as they affect the measurement. The test results and test circui t diagrams are given. The derived patterns will be implemented in a DO S-based expert system in combination with a data acquisition system wh ich is currently used in the field by more than 80 utilities.